Proposal: Remove mutable / immutable handler registry distinction

I'd like to float the idea of removing the mutable / immutable service registry distinction on `io.grpc.Server`, as I feel it is an unnecessary distinction. As far as I can tell, the sole purpose was a performance concern, but this shouldn't have bearing on the API surface. The methods to get it are Experimental, and only seem to be used for reflection. I believe reflection can still be efficient without knowing the difference.

This is not the same as the "fallback" registry, which is stable API, and IMO is a better distinction than mutable / immutable.

Thoughts @ejona86 ?